I’m somewhat... skeptical of the idea that one can just “misplace” 1.3 billion people.
That is exactly point of the analysis. The numbers that can be calculated by Western intelligence don't remotely match what the Chinese are posturing as true.
Chinese have an official GDP growth rate that almost all Western economists are skeptical of. Is it so hard to believe if the Chinese misstate on one official fact they won't misstate another? Why would they understate the population? It shows how impoverished that country really is.
Why would they overstate the growth rate? To keep the discontentment from building. Smith has been pretty dead on right with the material he has been willing to release.